Bcharre's two seasons are completely different trips. May to October is hiking weather: warm days, cool nights, the gorge and the cedar grove in full leaf, the road over to the Bekaa Valley open for traffic. June is probably ideal — the worst summer heat hasn't arrived in the mountains and there are still spring flowers above the tree line.
December to early April is the ski season. Bcharre fills up with Lebanese weekenders and there's a festive atmosphere in the cafes. The trade-off is that the higher trails are snowbound and the road through to Baalbek is closed.
Late March to early May and late October to November are the awkward seasons: skiing is over, hiking is wet and cold, and many guesthouses close. Avoid these months unless you want the town to yourself.